1. Field of the Invention
The present invention relates to magnetic sensors each having a free magnetic layer, a non-magnetic material layer, and a fixed magnetic layer, and more particularly, relates to a magnetic sensor fixing the magnetization of a fixed magnetic layer by uniaxial anisotropy thereof.
2. Description of the Related Art
Among magnetic sensors mounted in magnetic read and write devices, a spin-valve magnetic sensor exploiting a giant magnetoresistive (GMR) effect is the most widely used.
A spin-valve magnetic sensor comprises a laminate formed of a ferromagnetic film called a fixed magnetic layer and a ferromagnetic soft magnetic film called a free magnetic layer with a non-magnetic material film called a non-magnetic material layer interposed therebetween.
The magnetization of the free magnetic layer is oriented in a particular direction by a longitudinal bias magnetic field caused, for example, by hard bias layers made of a soft magnetic material. The magnetization direction of the free magnetic layer is sensitive and is changed by an external magnetic field supplied from a recording medium. On the other hand, the magnetization of the fixed magnetic layer is fixed in a direction intersecting the magnetization direction of the free magnetic layer.
Electrical resistance is changed by the relationship between the fixed magnetization direction of the fixed magnetic layer and the change in magnetization direction of the free magnetic layer. Using the change in voltage or change in current caused by this change in electrical resistance, a leak magnetic field from a recording medium may be detected.
Heretofore, the fixed magnetic layer is formed on an antiferromagnetic layer made of an antiferromagnetic material such as PtMn so that an exchange coupling magnetic field is generated therebetween, thereby fixing the magnetization of the fixed magnetic layer.
The exchange coupling magnetic field generated at an interface between the antiferromagnetic layer and the fixed magnetic layer can be sufficiently increased so as to prevent the change in magnetization direction of the fixed magnetic layer caused by a magnetic field applied in a production process or a leak magnetic field from a recording medium. In addition, since the antiferromagnetic layer itself generates no magnetic field outside, the magnetic sensor may be designed easily.
However, in order to generate a sufficiently large exchange coupling magnetic field at the interface between the antiferromagnetic layer and the fixed magnetic layer, the antiferromagnetic layer must have a thickness of approximately 200 Å.
An antiferromagnetic layer having a large thickness present in a laminate forming a magnetic sensor primarily causes a shunt loss in a sense current. In order to increase the recording density of a recording medium, the output of the magnetic sensor must be improved. However, the shunt loss described above causes deterioration in the magnetic sensor output, thereby inhibiting such an improvement.
In addition, for efficiently reading a record signal to be detected, shield layers each formed of a soft magnetic material are provided at a top and a bottom side of a magnetic sensor. To increase the recording density of the recording medium, it is necessary to decrease the distance between the top and the bottom shield layers. However, the large thickness of the structure due to the presence of the thick antiferromagnetic layer makes it difficult to decrease the distance between the top and the bottom shield layers.
Accordingly, as shown in FIG. 17, a magnetic sensor has been proposed in which an antiferromagnetic layer is not formed and in which the magnetization of a fixed magnetic layer is fixed by uniaxial anisotropy thereof.
In the magnetic sensor shown in FIG. 17, on a lower gap layer 1 made of an insulating material such as alumina, a multilayer film T is provided which is composed of an underlying layer 2; a fixed magnetic layer 3 having a synthetic ferrimagnetic structure composed of a first magnetic layer 3a and a second magnetic layer 3c with a non-magnetic interlayer 3b interposed therebetween; a non-magnetic material layer 4; a free magnetic layer 5; and a protective layer 6 in that order from the bottom side. In addition, on two sides 7 of the multilayer film T, bias underlying layers 8, hard bias layers 9, and electrode layers 10 are formed.
In the magnetic sensor shown in FIG. 17, an antiferromagnetic layer is not provided on the fixed magnetic layer 3, and by the uniaxial anisotropy of the fixed magnetic layer 3 itself, the magnetization thereof is fixed in a Y direction in the figure. Accordingly, since the shunt loss can be decreased as compared to that of a related magnetic sensor provided with an antiferromagnetic layer, a magnetic field detection output of the magnetic sensor can be improved by 20% to 30%. In addition, since the distance between the shield layers provided at the top and the bottom sides of the magnetic sensor can be decreased, the recording density of a recording medium can be increased.
A magnetic sensor as shown in FIG. 17 has been disclosed in Japanese Unexamined Patent Application Publication No. 8-7235 (pp. 8 and 9, and FIG. 5) and Japanese Unexamined Patent Application Publication No. 2000-113418 (pp. 7 and 8, and FIGS. 4 to 7)
In the magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 8-7235, on a buffer layer 62 of tantalum used as an underlayer, a pinned ferromagnetic layer 70 is provided. The pinned ferromagnetic layer 70 is a laminate composed of a first cobalt (Co) film 72 and a second cobalt (Co) film 74 with a ruthenium (Ru) film 73 interposed therebetween. The magnetizations of the first cobalt (Co) film 72 and the second cobalt (Co) film 74 are fixed by respective anisotropic magnetic fields thereof. The first cobalt (Co) film 72 and the second cobalt (Co) film 74 are antiferromagnetically coupled with each other and are magnetized antiparallel to each other.
However, it was found that, in the structure as that disclosed in Japanese Unexamined Patent Application Publication No. 8-7235 in which the Co films are provided on the buffer layer made of tantalum, the magnetization direction of the pinned ferromagnetic layer 70 cannot be appropriately fixed. This fact has also been disclosed in Japanese Unexamined Patent Application Publication No. 2000-113418.
The magnetic sensor disclosed in Japanese Unexamined Patent Application Publication No. 2000-113418 was made in order to solve the problem of Japanese Unexamined Patent Application Publication No. 8-7235. In this magnetic sensor, ferromagnetic films of a laminated ferrimagnetic fixed layer are formed using CoFe or CoFeNi, thereby improving the induced anisotropy.
In Japanese Unexamined Patent Application Publication No. 2000-113418, the structure in which an underlying layer made of tantalum (Ta) is provided under the laminated ferrimagnetic fixed layer is also described; however, according to experimental results (FIGS. 4 to 7 in Japanese Unexamined Patent Application Publication No. 2000-113418) obtained with and without the use of the Ta underlying layer, it has been shown that when a CoFe alloy is used for the ferromagnetic layer, the change in magnetoresistance and the coercive force are increased when the Ta underlying layer is not provided.
In order to increase the induced anisotropy of the laminated ferrimagnetic fixed layer, the use of a CoFe alloy for the ferromagnetic film and the use of a ferromagnetic layer having a positive magnetostriction constant are disclosed in Japanese Unexamined Patent Application Publication No. 2000-113418.
One factor in fixing the magnetization of a self-fixed magnetic layer is the uniaxial anisotropy derived from the magnetoelastic energy of the fixed magnetic layer. In particular, it is useful to optimize the magnetostriction of the fixed magnetic layer. However, in Japanese Unexamined Patent Application Publication No. 2000-113418, a mechanism for optimizing the magnetostriction of the fixed magnetic layer has not been discussed, nor has a particular structure for optimizing the magnetostriction of the fixed magnetic layer has been described. In addition, when the coercive force of the fixed magnetic layer can be increased besides the increase in magnetostriction, it is more preferable. When the coercive force is small, since the magnetization of the fixed magnetic layer fixed in a height direction is liable to be reversed by a mechanical stress (generated when a magnetic sensor impinges upon a surface of a recording medium or generated in manufacturing), reproduction properties become unstable, resulting in degradation in reliability.